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<h2>Events</h2>
<p>
There are three basic kinds of events that <b>GoJS</b> deals with:
<a>DiagramEvent</a>s, <a>InputEvent</a>s, and <a>ChangedEvent</a>s.
This page discusses the first two; see <a href="changedEvents.html">Changed Events</a> for the last kind of event.
</p>

<h3>Diagram Events</h3>
<p>
<a>DiagramEvent</a>s represent general user-initiated changes to a diagram.
You can register diagram event handlers by calling <a>Diagram.addDiagramListener</a>.
Each kind of diagram event is distinguished by its name.
</p>
<p>
Currently defined diagram event names include:
</p>
<ul>
  <li>"<b>AnimationStarting</b>", an animation is about to start.</li>
  <li>"<b>AnimationFinished</b>", an animation just completed.</li>
  <li>"<b>BackgroundSingleClicked</b>", when a mouse left-button single-click happened in the background of the Diagram, not on a Part</li>
  <li>"<b>BackgroundDoubleClicked</b>", when a mouse left-button double-click happened in the background of the Diagram, not on a Part</li>
  <li>"<b>BackgroundContextClicked</b>", when a mouse right-button single-click happened in the background of the Diagram, not on a Part</li>
  <li>"<b>ChangingSelection</b>", before the <a>Diagram.selection</a> collection changes</li>
  <li>"<b>ChangedSelection</b>", after the <a>Diagram.selection</a> collection has changed</li>
  <li>"<b>ClipboardChanged</b>", when <a>CommandHandler.copySelection</a> has changed the clipboard (but the Diagram is unmodified)</li>
  <li>"<b>ClipboardPasted</b>", when <a>CommandHandler.pasteSelection</a> has copied Parts from the clipboard into the Diagram; the new Parts are held in the <a>Diagram.selection</a> collection</li>
  <li>"<b>DocumentBoundsChanged</b>", when <a>Diagram.documentBounds</a> has changed value</li>
  <li>"<b>ExternalObjectsDropped</b>", when the <a>DraggingTool</a> has copied Parts from another diagram into the Diagram; the new Parts are held in the <a>Diagram.selection</a> collection</li>
  <li>"<b>InitialLayoutCompleted</b>", when the first diagram layout has completed after a major change to the diagram</li>
  <li>"<b>LayoutCompleted</b>", whenever a diagram layout has completed</li>
  <li>"<b>LinkDrawn</b>", when the <a>LinkingTool</a> has finished creating a new Link, which is the <a>DiagramEvent.subject</a></li>
  <li>"<b>LinkRelinked</b>", when the <a>RelinkingTool</a> has finished reconnecting an existing Link, which is the <a>DiagramEvent.subject</a></li>
  <li>"<b>LinkReshaped</b>", when the <a>LinkReshapingTool</a> has finished rerouting a Link, which is the <a>DiagramEvent.subject</a></li>
  <li>"<b>Modified</b>", the <a>Diagram.isModified</a> property has been set to a new value; do not modify the Diagram or its Model in the event listener</li>
  <li>"<b>PartCreated</b>", when the <a>ClickCreatingTool</a> has inserted a new Part, which is the <a>DiagramEvent.subject</a></li>
  <li>"<b>PartResized</b>", when the <a>ResizingTool</a> has resized a GraphObject, which is the <a>DiagramEvent.subject</a></li>
  <li>"<b>PartRotated</b>", when the <a>RotatingTool</a> has rotated a GraphObject, which is the <a>DiagramEvent.subject</a></li>
  <li>"<b>ObjectSingleClicked</b>", when a mouse left-button single-click happened on a GraphObject, which is the <a>DiagramEvent.subject</a></li>
  <li>"<b>ObjectDoubleClicked</b>", when a mouse left-button double-click happened on a GraphObject, which is the <a>DiagramEvent.subject</a></li>
  <li>"<b>ObjectContextClicked</b>", when a mouse right-button single-click happened on a GraphObject, which is the <a>DiagramEvent.subject</a></li>
  <li>"<b>SelectionMoved</b>", when the <a>DraggingTool</a> has moved some Parts; the moved Parts are held in the <a>Diagram.selection</a> collection</li>
  <li>"<b>SelectionCopied</b>", when the <a>DraggingTool</a> has copied some Parts; the copied Parts are held in the <a>Diagram.selection</a> collection</li>
  <li>"<b>SelectionDeleting</b>", before <a>CommandHandler.deleteSelection</a> deletes the <a>Diagram.selection</a> Parts; set <a>DiagramEvent.cancel</a> to true in order to cancel the deletion</li>
  <li>"<b>SelectionDeleted</b>", after <a>CommandHandler.deleteSelection</a> has deleted the <a>Diagram.selection</a>, which is now empty</li>
  <li>"<b>SelectionGrouped</b>", after <a>CommandHandler.groupSelection</a> has create a new Group from the <a>Diagram.selection</a>, which now holds the new Group</li>
  <li>"<b>SelectionUngrouped</b>", after <a>CommandHandler.ungroupSelection</a> has removed the selected Group and kept the member Parts as the new <a>Diagram.selection</a></li>
  <li>"<b>SubGraphCollapsed</b>", when <a>CommandHandler.collapseSubGraph</a> has collapsed selected Groups, making their member Parts not visible</li>
  <li>"<b>SubGraphExpanded</b>", when <a>CommandHandler.expandSubGraph</a> has expanded selected Groups, making their member Parts visible</li>
  <li>"<b>TextEdited</b>", when the <a>TextEditingTool</a> has completed in-place text editing; the modified TextBlock is the <a>DiagramEvent.subject</a></li>
  <li>"<b>TreeCollapsed</b>", when <a>CommandHandler.collapseTree</a> has collapsed selected Nodes with subtrees, making the subtree nodes and links not visible</li>
  <li>"<b>TreeExpanded</b>", when <a>CommandHandler.expandTree</a> has expanded selected Nodes with subtrees, making the subtree nodes and links visible</li>
  <li>"<b>ViewportBoundsChanged</b>", when <a>Diagram.viewportBounds</a> has changed value</li>
</ul>
For more details, please read the documentation for <a>DiagramEvent</a>.

<p>
DiagramEvents do not necessarily correspond to mouse events or keyboard events or touch events.
Nor do they necessarily correspond to changes to the diagram's model --
for tracking such changes, use <a>Model.addChangedListener</a> or <a>Diagram.addModelChangedListener</a>.
DiagramEvents only occur because the user did something, perhaps indirectly.
</p>
<p>
<p>
In addition to the DiagramEvent listeners there are also circumstances where detecting such changes is common
enough to warrant having properties that are event handlers.
Because these events do not necessarily correspond to any particular input or diagram event,
these event handlers have custom arguments that are specific to the situation.
</p>
<p>
One very common such event property is <a>GraphObject.click</a>, which if non-null is a function that is called
whenever the user clicks on that object.
This is most commonly used to specify behavior for "Button"s, but it and the other "click" event properties,
"doubleClick" and "contextClick", can be useful on any GraphObject.
</p>
<p>
Another common event property is <a>Part.selectionChanged</a>,
which (if non-null) is called whenever <a>Part.isSelected</a> changes.
In this case the event hander function is passed a single argument, the Part.
There is no need for additional arguments because the function can check the current value of <a>Part.isSelected</a> to decide what to do.
</p>
</p>
<p>
Model <a>ChangedEvent</a>s are more complete and reliable than depending on <a>DiagramEvent</a>s.
For example, the "LinkDrawn" DiagramEvent is not raised when code adds a link to a diagram.
That DiagramEvent is only raised when the user draws a new link using the <a>LinkingTool</a>.
Furthermore the link has not yet been routed, so <a>Link.points</a> will not have been computed.
In fact, creating a new link may invalidate a <a>Layout</a>, so all of the nodes may be moved in the near future.
</p>

<p class="box bg-danger">
Sometimes you want to update a database as the user makes changes to a diagram.
Usually you will want to implement a <a>Model</a> <a>ChangedEvent</a> listener,
by calling <a>Model.addChangedListener</a> or <a>Diagram.addModelChangedListener</a>,
that notices the changes to the model and decides what to record in the database.
See the discussion of <a href="changedEvents.html">Changed Events</a> and the <a href="../samples/UpdateDemo.html">Update Demo</a>.
</p>

<p>
This example demonstrates handling several diagram events: <b>"ObjectSingleClicked"</b>,
<b>"BackgroundDoubleClicked"</b>, <b>"ClipboardPasted"</b>, and <b>"SelectionDeleting"</b>.
</p>
<pre data-language="javascript" id="diagramEvents">
  function showMessage(s) {
    document.getElementById("diagramEventsMsg").textContent = s;
  }

  diagram.addDiagramListener("ObjectSingleClicked",
      function(e) {
        var part = e.subject.part;
        if (!(part instanceof go.Link)) showMessage("Clicked on " + part.data.key);
      });

  diagram.addDiagramListener("BackgroundDoubleClicked",
      function(e) { showMessage("Double-clicked at " + e.diagram.lastInput.documentPoint); });

  diagram.addDiagramListener("ClipboardPasted",
      function(e) { showMessage("Pasted " + e.diagram.selection.count + " parts"); });

  diagram.addDiagramListener("SelectionDeleting",
      function(e) {
        if (e.diagram.selection.count > 1) {
          e.cancel = true;
          showMessage("Cannot delete multiple selected parts");
        }
      });

  var nodeDataArray = [
    { key: "Alpha" },
    { key: "Beta", group: "Omega" },
    { key: "Gamma", group: "Omega" },
    { key: "Omega", isGroup: true },
    { key: "Delta" }
  ];
  var linkDataArray = [
    { from: "Alpha", to: "Beta" },  // from outside the Group to inside it
    { from: "Beta", to: "Gamma" },  // this link is a member of the Group
    { from: "Omega", to: "Delta" }  // from the Group to a Node
  ];
  diagram.model = new go.GraphLinksModel(nodeDataArray, linkDataArray);
</pre>
<script>goCode("diagramEvents", 600, 200)</script>
<span id="diagramEventsMsg" style="color: red">(message)</span>


<h3>Input Events</h3>
<p>
When a low-level HTML DOM event occurs, <b>GoJS</b> canonicalizes the keyboard/mouse/touch event information
into a new <a>InputEvent</a> that can be passed to various event-handling methods and saved for later examination.
</p>
<p>
An InputEvent keeps the <a>InputEvent.key</a> for keyboard events,
the <a>InputEvent.button</a> for mouse events,
the <a>InputEvent.viewPoint</a> for mouse and touch events,
and <a>InputEvent.modifiers</a> for keyboard and mouse events.
</p>
<p>
The diagram's event handlers also record the <a>InputEvent.documentPoint</a>,
which is the <a>InputEvent.viewPoint</a> in document coordinates at the time of the mouse event,
and the <a>InputEvent.timestamp</a>, which records the time that the event occurred in milliseconds.
</p>
<p>
The InputEvent class also provides many handy properties for particular kinds of events.
Examples include <a>InputEvent.control</a> (if the control key had been pressed) and
<a>InputEvent.left</a> (if the left/primary mouse button was pressed).
</p>
<p>
Some tools find the "current" <a>GraphObject</a> at the mouse point.
This is remembered as the <a>InputEvent.targetObject</a>.
</p>

<h3>Higher-level input events</h3>
<p>
Some tools detect a sequence of input events to compose somewhat more abstract user events.
Examples include "click" (mouse-down-and-up very close to each other) and "hover" (motionless mouse for some time).
The tools will call an event handler (if there is any) for the current <a>GraphObject</a> at the mouse point.
The event handler is held as the value of a property on the object.
It then also "bubbles" the event up the chain of <a>GraphObject.panel</a>s until it ends with a <a>Part</a>.
This allows a "click" event handler to be declared on a <a>Panel</a> and have it apply even if the click actually happens on an element deep inside the panel.
If there is no object at the mouse point, the event occurs on the diagram.
</p>
<p>
Click-like event properties include <a>GraphObject.click</a>, <a>GraphObject.doubleClick</a>, and <a>GraphObject.contextClick</a>.
They also occur when there is no GraphObject -- the event happened in the diagram's background:
<a>Diagram.click</a>, <a>Diagram.doubleClick</a>, and <a>Diagram.contextClick</a>.
These are all properties that you can set to a function that is the event handler.
These events are caused by both mouse events and touch events.
</p>
<p>
Mouse-over-like event properties include <a>GraphObject.mouseEnter</a>, <a>GraphObject.mouseOver</a>, and <a>GraphObject.mouseLeave</a>.
But only <a>Diagram.mouseOver</a> applies to the diagram.
</p>
<p>
Hover-like event properties include <a>GraphObject.mouseHover</a> and <a>GraphObject.mouseHold</a>.
The equivalent diagram properties are <a>Diagram.mouseHover</a> and <a>Diagram.mouseHold</a>.
</p>
<p>
There are also event properties for dragging operations: <a>GraphObject.mouseDragEnter</a>, <a>GraphObject.mouseDragLeave</a>, and <a>GraphObject.mouseDrop</a>.
These apply to stationary objects, not the objects being dragged.
And they also occur when dragging by touch events, not just mouse events.
</p>

<p>
This example demonstrates handling three higher-level input events:
clicking on nodes and entering/leaving groups.
</p>
<pre data-language="javascript" id="inputEvents">
  function showMessage(s) {
    document.getElementById("inputEventsMsg").textContent = s;
  }

  diagram.nodeTemplate =
    $(go.Node, "Auto",
      $(go.Shape, "Ellipse", { fill: "white" }),
      $(go.TextBlock,
        new go.Binding("text", "key")),
      { click: function(e, obj) { showMessage("Clicked on " + obj.part.data.key); } }
    );

  diagram.groupTemplate =
    $(go.Group, "Vertical",
      $(go.TextBlock,
        { alignment: go.Spot.Left, font: "Bold 12pt Sans-Serif" },
        new go.Binding("text", "key")),
      $(go.Panel, "Auto",
        $(go.Shape, "RoundedRectangle",
          { name: "SHAPE",
            parameter1: 14,
            fill: "rgba(128,128,128,0.33)" }),
        $(go.Placeholder, { padding: 5 })
      ),
      { mouseEnter: function(e, obj, prev) {  // change group's background brush
            var shape = obj.part.findObject("SHAPE");
            if (shape) shape.fill = "red";
          },
        mouseLeave: function(e, obj, next) {  // restore to original brush
            var shape = obj.part.findObject("SHAPE");
            if (shape) shape.fill = "rgba(128,128,128,0.33)";
          } });

  var nodeDataArray = [
    { key: "Alpha" },
    { key: "Beta", group: "Omega" },
    { key: "Gamma", group: "Omega" },
    { key: "Omega", isGroup: true },
    { key: "Delta" }
  ];
  var linkDataArray = [
    { from: "Alpha", to: "Beta" },  // from outside the Group to inside it
    { from: "Beta", to: "Gamma" },  // this link is a member of the Group
    { from: "Omega", to: "Delta" }  // from the Group to a Node
  ];
  diagram.model = new go.GraphLinksModel(nodeDataArray, linkDataArray);
</pre>
<script>goCode("inputEvents", 600, 200)</script>
<span id="inputEventsMsg" style="color: red">(message)</span>


<h3>Clicking and Selecting</h3>
<p>
This example demonstrates both the "click" and the "selectionChanged" events:
</p>
<pre data-language="javascript" id="changeMethods">
  function showMessage(s) {
    document.getElementById("changeMethodsMsg").textContent = s;
  }

  diagram.nodeTemplate =
    $(go.Node, "Auto",
      { selectionAdorned: false },
      $(go.Shape, "Ellipse", { fill: "white" }),
      $(go.TextBlock,
        new go.Binding("text", "key")),
      {
        click: function(e, obj) { showMessage("Clicked on " + obj.part.data.key); },
        selectionChanged: function(part) {
            var shape = part.elt(0);
            shape.fill = part.isSelected ? "red" : "white";
          }
      }
    );

  var nodeDataArray = [
    { key: "Alpha" }, { key: "Beta" }, { key: "Gamma" }
  ];
  var linkDataArray = [
    { from: "Alpha", to: "Beta" },
    { from: "Beta", to: "Gamma" }
  ];
  diagram.model = new go.GraphLinksModel(nodeDataArray, linkDataArray);
</pre>
<script>goCode("changeMethods", 600, 200)</script>
<span id="changeMethodsMsg" style="color: red">(message)</span>
<p>
Try Ctrl-A to select everything.
Note the distinction between the <a>GraphObject.click</a> event property and the <a>Part.selectionChanged</a> event property.
Both are methods that get called when something has happened to the node.
The <a>GraphObject.click</a> occurs when the user clicks on the node, which happens to select the node.
But the <a>Part.selectionChanged</a> occurs even when there is no click event or even any mouse event --
it was due to a property change to the node.
</p>

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